A hard disk drive includes a disk on which data is stored, a spindle motor which rotates the disk, a read/write head, and a voice coil motor (VCM) to move the head over the disk for reading/writing from/to the disk surface. When the disk is not rotating, the VCM moves the head away from the disk area. When the disk is rotating, and read/write operations are in progress, the head, which is carried on a VCM arm, is positioned above the data storage surface of the disk. When the disk is not rotating, the head is moved away from the disk in order to prevent any damage to the disk.
Conventional systems generally use two methods or modes for controlling the velocity of VCMs, continuous and discontinuous mode.
Conventional discontinuous mode methods generally drive the VCM motor using Pulse Width Modulation (PWM). During the PWM off-state, the back electromagnetic field (BEMF) voltage appearing across the VCM motor is sensed. The BEMF voltage information is used to determine the power required to drive the VCM motor in the next cycle. Conventional methods using discontinuous modes mandate that the PWM frequency be in the audible range. This is typically due to the L/R constant of the inductive motor and requirements for sufficient off-time to sense the BEMF effectively.
For conventional continuous mode methods, a constant voltage is applied across the VCM motor. Thus, the motor maximum velocity is limited. Such methods do not control the velocity effectively. Often times, the voltage includes the BEMF and voltage across the motor resistance (Rm). Another conventional method senses the motor current through the sense resistor (Rs) and voltage across the VCM motor. This information is then used to calculate the VCM motor resistance and the actual BEMF.
It is generally important that there exists a precise control over the velocity of the actuator in a Hard Disk Drive (HDD). Most conventional methods control the Voice Coil Motor(VCM) during the load/unloading operation of HDDs with ramp load features. Such control enables the actuator to transit onto or off the ramp without damaging the HDD by preventing crashes against the media or ramp in high speeds.
To achieve good velocity control, it is generally important to extract the VCM back electromagnetic field (BEMF) voltage and the motor resistance accurately. This is typically very difficult because motor resistances generally vary with respect to temperature and the torque required as the arm is brought up and down the ramp.
There is therefore a need for improved systems and methods for controlling voice coil motors.